Navigation instruments used to determine the latitude position of surface vessels such as ships or the like are old in the art. One such instrument is shown by McCallister, U.S. Pat. No. 938,780. The instrument disclosed includes a base upon which are disposed a pair of scales. One scale denotes the degrees of latitude, while the other scale is divided into the number of days included between the longest day of the year and the shortest day of the year. An arm is perpendicular to the base and moveably positioned within a slot at one end of the base along the length of the scale containing the number of days of the year. By orienting the base in a north-south direction, and by moving the arm opposite a position on the scale designating the day on which the reading is taken, the end of the shadow cast by the arm on the scale having the degrees of latitude indicates the latitude position of the surface vessel. The disclosed instrument corrects for the declination of the sun by movement of an arm which is perpendicular to the base. The instrument therefore requires placement of the base on a stationary planer surface of the vessel located in a meridian plane before the latitude of the vessel can be determined. In the absence of such a planar surface, the accuracy of any subsequent latitude reading is questionable.
Instruments disclosed in the prior art used to aid navigation employ various means which cast shadows to determine the altitude of a celestial body such as the sun or the like. Typical instruments are shown in Hewitt, U.S. Pat. No. 728,915, and France, U.S. Pat. No. 2,904,889. In general, the disclosed devices must be placed on a stationary horizontal planer surface in order to accurately determine the height of the celestial body. The points at which the shadows cast by the vertical member of the disclosed instruments intersect reference scales are used in complicated trigonometric formulas, the solutions of which are needed to accurately determine the height of the celestial body.
The present invention overcomes the problems and limitations of the nautical and navigational instruments disclosed in the prior art. The disclosed invention does not require placement on a planar surface of the vessel to determine the latitude of the vessel for the reason that the invention corrects for the declination of the sun by use of a pendulum disposed on a vertical member of the instrument. The pendulum is aligned with the day the reading is taken by manually orienting the invention in the noon day sun until one end of the pendulum points to the current day date. The shadow cast by the vertical member when the pendulum has been so aligned impinges upon a scale having latitude markings. By noting the point at which the shadow strikes the scale, the latitude of the vessel is directly determined without the use of trigonometric formulas and time consuming, complex calculations.